I'm just wondering if this sounds right. It seems like the basis of jazz harmony makes use of seventh chord and classical music the basis is triads?
I think you’re on the wrong track with this. It’s not about the kinds of chords used. It’s more about what each chord means in the context and how they relate to each other.
Music from the classical period can be effectively analyzed by viewing it as functional harmony, while much (not all) jazz music can be understood by looking at tritone substitution.
The fact that 7th chords often have tritones and most triads do not is as close as it gets to being about the types of chords. Triads are used in jazz and 7th chords are used in classical music, so there’s not a clear distinction in that sense.
As an extreme simplification, it's true that most jazz music has many more 7th chords than classical. But I wouldn't argue that classical is based on triads, as that's extremely narrow-minded and ignores the extremely important polyphonic textures of classical music. No classical musician would support that statement. Jazz, in turn, is not necessarily based off the 7th chords. Extended chords are built off the 7th chords, but...
"Jazz is more than just a stack of thirds." -user45266
To argue that jazz is based off of 7th chords seems more like the view of someone looking at some jazz as an outsider, noticing the 7th chords in the sheets, and overextending the 7th chords' importance. Jazz has many hugely important defining characteristics, for example improvisation, that can't be explained as 7th chords.
In short, the statement you have made is representative of an astute observation: Jazz music uses more 7th chords than classical music, and in more varying contexts. But the position you offer is indefensible when faced with the many aspects of each genre that cannot be explained solely by chord qualities.
Supposing the chord type to be the basis will give the wrong understanding especially when looking at classical music.
If you think of it as: 'jazz stacks up 3 thirds on scale degrees to make seventh chords, but classical only stacks up 2 thirds,' it will lead to misunderstanding.
Yes, jazz uses 7th chords.
But so does classical music. Dominant 7th, minor 7th, half-diminished 7ths, full diminished sevenths, and major 7ths are all used in classical style. The major 7th is a bit of a rarity, but it is used.
The important distinction regarding 7th chords is: classical music treats the interval of the seventh as a dissonance which (normally, but not always) needs to be resolved.
- Jazz 7th chord voicings can move freely without obligatory resolution, essentially the seventh is a consonance. Classical resolves the sevenths, because it's a dissonance.
- Jazz can use a 7th chord as a tonic chord, because the chord is consonant. Classical music only uses major/minor triads as tonics. Classical doesn't use 7th chords as tonics, because you don't use a dissonance as a tonic chord.
That's really the main difference. In classical music the tonic won't be a seventh chord.
Otherwise a lot is the same. Root progression by descending fifth is important in both styles.
The tri-tone substitution is the same as an augmented sixth chord in classical.
A chromatic circle of fifths sequence of all dominant seventh chords is more common in jazz than in classical music, but it happens in both styles.
If you really want to put your finger on the tonal difference between jazz and classical, I would say look at the treatment of the scale mediant. Jazz (blues) plays with the major/minor third of the scale in a unique way that classical style does not use. Sometimes you will get the enharmonic equivalent of that minor third in classical, but it's conception is different. In jazz it's a
b^3 over a major chord, but in classical it would be a
#^2 non-chord tone. I think that minor third is truly the basis of traditional jazz tonality.